1. Field of the Invention
The present invention relates to the manufacture of a forged connecting rod, and especially of a forged connecting rod for an internal combustion engine.
2. Discussion of the Background
Forged connecting rods, in particular forged connecting rods for an internal combustion engine, especially when they are made of steel, are manufactured by a process which comprises hot forging of a connecting rod rough forging, a heat treatment and a machining operation.
The forging includes several steps which are, in general, the manufacture of a slug by the shaping rolling of a billet treated beforehand to a suitable temperature, the drop forging or closed-die forging of the slug, the cutting-off of the forging flash or its trimming, and a punching step.
The heat treatment, which may be carried out at a later stage or while the rough forging is still hot, depends especially on the grade of steel used and on the desired mechanical properties.
When it is carried out at a later stage, the heat treatment may be a normalization intended to obtain a pearlitic or ferrito-pearlitic structure. This is, in particular, the case when separable connecting rods are manufactured, that is to say when connecting rods are manufactured whose big end may be separated into two parts by brittle fracture. The heat treatment at a later stage may also be a hardening step followed by an annealing step, the hardening being intended to obtain a mainly martensitic structure whose mechanical properties are very high.
When the heat treatment is carried out while the forging is still hot, which assumes that the temperature at the end of the forging operation is high enough, it always includes a controlled cooling operation, either cooling sufficiently slowly to obtain a pearlitic or ferrito-pearlitic structure or sufficiently rapidly to obtain a mainly bainitic structure. When the desired structure is mainly bainitic, the controlled cooling may include a temperature hold intended to cause a substantially isothermal transformation. It may also include reheating to a temperature below approximately 600.degree. C. in order to produce an annealing effect. The annealing may also be carried out at a later stage.
The machining operation mainly includes the trueing of the side faces and the machining of the bores in the big end and in the small end of the connecting rod, the separation of the big end into two parts and the drilling of holes intended to receive the fixing bolts for the two parts of the big end. The big end is separated into two parts either by machining or, when the big end is separable, by brittle fracture under an impact.
In order for the machining operation to be carried out under the proper precision conditions, it is necessary to premachine the reference surfaces at the periphery of the rough forging so as to suitably position it on the machining machines, and this operation is tricky.
Apart from the machining difficulties, this technique also has a drawback, in the case of mass production, of leading to too great a variation in the dimensions and in the weight of the connecting rods to be able to fit them into engines indiscriminately. When the weights of the connecting rods in the same engine vary too much, the engine is poorly balanced. Therefore, before fitting into engines, the connecting rods are sorted in order to group them into narrow weight classifications. These grouping operations require accurate weighing and complicate the manufacture.
In order to improve the geometrical precision of the connecting rod rough forgings, it has been proposed, in German Utility Model DE 43 29 371, to replace the closed-die forging of the slug with forging of the slug in a closed die, by plating between two inserts provided with impressions. The rough forging obtained includes a shank, a big end and a small end; the big end and the small end each include two bore initiators separated by a web. At the end of forging, the relative movement of the two inserts is stopped when the volume of the space delimited by the two inserts and by the die is equal to the volume of metal of the slug. The geometrical precision and the precision in the weight of the rough forging are then directly determined by the precision in the weight of the slug. In order to obtain a precision rough forging, it is necessary to use a precision slug which can only be obtained by machining, which operation is very expensive for the manufacture of a slug. In addition, the forging of the slug is carried out with large deformations of the metal, the deformation ratios are several tens of %, which lead to rapid wear of the inserts unfavorable to mass production.